In general, Amplitude Modulation is a process of combining a modulating signal with a carrier signal to produce an amplitude modulated signal. In the frequency spectrum, the amplitude modulated signal appears as discrete upper and lower sidebands, which are the sum and difference frequencies of the carrier and the modulating signal. The frequencies above the carrier frequency are collectively referred to as the “upper sideband” and the frequencies below the carrier frequency are referred to as the “lower sideband”. The upper and lower side bands include the same amount of information.
Transmission of both the upper and lower sidebands, with the carrier suppressed/reduced, is referred to as DSB (double sideband) transmission, and requires significant bandwidth. The transmission of one of the sidebands is referred to as SSB (single sideband) transmission, which is accomplished by suppressing the carrier and eliminating one sideband using a filter, requires less bandwidth than DSB. Voice or musical signals may be transmitted using only SSB modulation, because such signals contain a small number of signal components in a low frequency band. However, a TV signal has a significant number of signal components at a still lower frequency band, and thus, transmission of a TV signal using SSB modulation results in a great loss of information. For this reason, vestigial sideband (VSB) modulation is used to transmit TV signals, which is compromise between DSB and SSB, wherein portions of one of the redundant sidebands are removed to form a vestigial sideband signal.
Typically, a ring modulator or a balanced modulator is implemented for performing SSB modulation. A balanced modulator or ring modulator receives as input a carrier frequency (LO signal) and a modulating signal and generates a double sideband signal with a suppressed carrier. To obtain an SSB signal for SSB transmission, a balanced modulator is used to suppress/reduce the carrier signal and the output of the balanced modulator is filtered using a band-pass filter to remove either the upper or lower sideband to produce an IF (intermediate frequency signal).
On the receiving side, if a signal having an intermediate frequency (IF) as the frequency of a carrier wave, is input to an up-conversion mixer in the receiver, a local oscillating frequency LO allows the mixer to output two components such as LO+IF and LO−IF. If the LO−IF is a desired signal, namely, the data, the LO+IF is an undesired image signal that should be eliminated.
FIG. 1 is a circuit diagram of a conventional SSB mixer. Referring to FIG. 1, the conventional SSB mixer includes a first mixer 10, a second mixer 11, a phase-shifting unit 12, and an adder 13. A IF (intermediate frequency) signal having a frequency of cos (ωIFt) and a signal having a frequency of cos (ωLOt) (which is an LO signal that is phase-shifted by 90° by the phase-shifting unit 12) are input to the first mixer 10. Then, the first mixer 10 outputs signals having frequencies of ωLO+ωIF and ωLO−ωIF.
A signal having a frequency of sin (ωIFt) (which is an out-of-phase component of the IF signal) and a local oscillating frequency sin (ωLOt) are input to the second mixer 11. The second mixer 11 outputs signals having a frequencies of −(ωLO+ωIF) and ωLO−ωIF.
The adder 13 adds the outputs of the mixers 10 and 11, and then outputs only one frequency component ωLO−ωIF.
The SSB mixer of FIG. 1 has disadvantages in that the mixer requires a 90° phase-shifting device at a high frequency. That is, unless the phase of a signal is displaced precisely by 90°, the image component is not completely removed. Accordingly, the use of such a conventional SSB mixer exhibits rejection characteristics of about 30–40 dB with respect to the image component.